python - 如何创建正确垃圾收集的自定义生成器类
问题描述
我正在尝试在 Python 中编写一个充当生成器对象的类,特别是当它被垃圾收集时,它会.close()
被调用。这很重要,因为这意味着当生成器中断时,我可以确保它会自行清理,例如关闭文件或释放锁。
下面是一些解释性代码:如果你中断了一个生成器,那么当它被垃圾回收时,Python 会调用.close()
生成器对象,这会向GeneratorExit
生成器抛出一个错误,可以捕获该错误以进行清理,如下所示:
from threading import Lock
lock = Lock()
def CustomGenerator(n, lock):
lock.acquire()
print("Generator Started: I grabbed a lock")
try:
for i in range(n):
yield i
except GeneratorExit:
lock.release()
print("Generator exited early: I let go of the lock")
raise
print("Generator finished successfully: I let go of the lock")
for i in CustomGenerator(100, lock):
print("Received ", i)
time.sleep(0.02)
if i==3:
break
if not lock.acquire(blocking=False):
print("Oops: Finished, but lock wasn't released")
else:
print("Finished: Lock was free")
lock.release()
Generator Started: I grabbed a lock
Received 0
Received 1
Received 2
Received 3
Generator exited early: I let go of the lock
Finished: Lock was free
但是,如果您尝试通过继承 from 来实现自己的生成器对象collections.abc.Generator
,Python 似乎没有注意到它应该在收集对象时调用 close:
from collections.abc import Generator
class CustomGeneratorClass(Generator):
def __init__(self, n, lock):
super().__init__()
self.lock = lock
self.lock.acquire()
print("Generator Class Initialised: I grabbed a lock")
self.n = n
self.c = 0
def send(self, arg):
value = self.c
if value >= self.n:
raise StopIteration
self.c += 1
return value
def throw(self, type, value=None, traceback=None):
print("Exception Thrown in Generator: I let go of the lock")
self.lock.release()
raise StopIteration
for i in CustomGeneratorClass(100, lock):
print("Received ", i)
time.sleep(0.02)
if i==3:
break
if not lock.acquire(blocking=False):
print("Oops: Finished, but lock wasn't released")
else:
print("Finished: Lock was free")
lock.release()
Generator Class Initialised: I grabbed a lock
Received 0
Received 1
Received 2
Received 3
Oops: Finished, but lock wasn't released
我认为继承Generator
足以让 python 相信我的 CustomGeneratorClass 是一个生成器,并且应该.close()
在垃圾收集时调用它。
我认为这与“生成器对象”是某种特殊的事实有关Generator
:
from types import GeneratorType
c_gen = CustomGenerator(100)
c_gen_class = CustomGeneratorClass(100)
print("CustomGenerator is a Generator:", isinstance(c_gen, Generator))
print("CustomGenerator is a GeneratorType:",isinstance(c_gen, GeneratorType))
print("CustomGeneratorClass is a Generator:",isinstance(c_gen_class, Generator))
print("CustomGeneratorClass is a GeneratorType:",isinstance(c_gen_class, GeneratorType))
CustomGenerator is a Generator: True
CustomGenerator is a GeneratorType: True
CustomGeneratorClass is a Generator: True
CustomGeneratorClass is a GeneratorType: False
我可以制作一个用户定义的类对象GeneratorType
吗?
关于python如何决定调用什么,我有什么不明白的地方.close()
吗?
如何确保.close()
在我的自定义生成器上调用它?
这个问题不是How to write a generator class的重复。对于实际制作生成器类,该问题的公认答案确实推荐了我在这里尝试的结构,这是一个生成器类,但没有正确收集垃圾,如上面的代码所示。
解决方案
PEP342,状态:
[generator].__del__()
是[generator].close()
. 这将在生成器对象被垃圾收集时调用......
collections.abc中的 Generator 类没有实现__del__
,它的超类或元类也没有。
将此实现添加__del__
到问题中的类会导致锁被释放:
class CustomGeneratorClass(Generator):
...
def __del__(self):
self.close()
输出:
Generator Class Initialised: I grabbed a lock
Recieved 0
Recieved 1
Recieved 2
Recieved 3
Exception Thrown in Generator: I let go of the lock
Finished: Lock was free
警告:
我对 Python 中对象终结的复杂性没有经验,因此应该对这个建议进行批判性检查,并进行破坏测试。特别是,应考虑语言参考__del__
中的警告。
更高级别的解决方案是在上下文管理器中运行生成器
with contextlib.closing(CustomGeneratorClass(100, lock)):
# do stuff
但这很麻烦,并且依赖于代码的用户记住这样做。
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